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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

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 * accompanied this code).

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 *

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package java.util;

import java.io.IOException;

import java.io.InvalidObjectException;

import java.io.Serializable;

import java.lang.reflect.ParameterizedType;

import java.lang.reflect.Type;

import java.util.function.BiConsumer;

import java.util.function.BiFunction;

import java.util.function.Consumer;

import java.util.function.Function;

import jdk.internal.misc.SharedSecrets;

/**

 * Hash table based implementation of the {@code Map} interface.  This

 * implementation provides all of the optional map operations, and permits

 * {@code null} values and the {@code null} key.  (The {@code HashMap}

 * class is roughly equivalent to {@code Hashtable}, except that it is

 * unsynchronized and permits nulls.)  This class makes no guarantees as to

 * the order of the map; in particular, it does not guarantee that the order

 * will remain constant over time.

 *

 * <p>This implementation provides constant-time performance for the basic

 * operations ({@code get} and {@code put}), assuming the hash function

 * disperses the elements properly among the buckets.  Iteration over

 * collection views requires time proportional to the "capacity" of the

 * {@code HashMap} instance (the number of buckets) plus its size (the number

 * of key-value mappings).  Thus, it's very important not to set the initial

 * capacity too high (or the load factor too low) if iteration performance is

 * important.

 *

 * <p>An instance of {@code HashMap} has two parameters that affect its

 * performance: <i>initial capacity</i> and <i>load factor</i>.  The

 * <i>capacity</i> is the number of buckets in the hash table, and the initial

 * capacity is simply the capacity at the time the hash table is created.  The

 * <i>load factor</i> is a measure of how full the hash table is allowed to

 * get before its capacity is automatically increased.  When the number of

 * entries in the hash table exceeds the product of the load factor and the

 * current capacity, the hash table is <i>rehashed</i> (that is, internal data

 * structures are rebuilt) so that the hash table has approximately twice the

 * number of buckets.

 *

 * <p>As a general rule, the default load factor (.75) offers a good

 * tradeoff between time and space costs.  Higher values decrease the

 * space overhead but increase the lookup cost (reflected in most of

 * the operations of the {@code HashMap} class, including

 * {@code get} and {@code put}).  The expected number of entries in

 * the map and its load factor should be taken into account when

 * setting its initial capacity, so as to minimize the number of

 * rehash operations.  If the initial capacity is greater than the

 * maximum number of entries divided by the load factor, no rehash

 * operations will ever occur.

 *

 * <p>If many mappings are to be stored in a {@code HashMap}

 * instance, creating it with a sufficiently large capacity will allow

 * the mappings to be stored more efficiently than letting it perform

 * automatic rehashing as needed to grow the table.  Note that using

 * many keys with the same {@code hashCode()} is a sure way to slow

 * down performance of any hash table. To ameliorate impact, when keys

 * are {@link Comparable}, this class may use comparison order among

 * keys to help break ties.

 *

 * <p><strong>Note that this implementation is not synchronized.</strong>

 * If multiple threads access a hash map concurrently, and at least one of

 * the threads modifies the map structurally, it <i>must</i> be

 * synchronized externally.  (A structural modification is any operation

 * that adds or deletes one or more mappings; merely changing the value

 * associated with a key that an instance already contains is not a

 * structural modification.)  This is typically accomplished by

 * synchronizing on some object that naturally encapsulates the map.

 *

 * If no such object exists, the map should be "wrapped" using the

 * {@link Collections#synchronizedMap Collections.synchronizedMap}

 * method.  This is best done at creation time, to prevent accidental

 * unsynchronized access to the map:<pre>

 *   Map m = Collections.synchronizedMap(new HashMap(...));</pre>

 *

 * <p>The iterators returned by all of this class's "collection view methods"

 * are <i>fail-fast</i>: if the map is structurally modified at any time after

 * the iterator is created, in any way except through the iterator's own

 * {@code remove} method, the iterator will throw a

 * {@link ConcurrentModificationException}.  Thus, in the face of concurrent

 * modification, the iterator fails quickly and cleanly, rather than risking

 * arbitrary, non-deterministic behavior at an undetermined time in the

 * future.

 *

 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed

 * as it is, generally speaking, impossible to make any hard guarantees in the

 * presence of unsynchronized concurrent modification.  Fail-fast iterators

 * throw {@code ConcurrentModificationException} on a best-effort basis.

 * Therefore, it would be wrong to write a program that depended on this

 * exception for its correctness: <i>the fail-fast behavior of iterators

 * should be used only to detect bugs.</i>

 *

 * <p>This class is a member of the

 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">

 * Java Collections Framework</a>.

 *

 * @param <K> the type of keys maintained by this map

 * @param <V> the type of mapped values

 *

 * @author  Doug Lea

 * @author  Josh Bloch

 * @author  Arthur van Hoff

 * @author  Neal Gafter

 * @see     Object#hashCode()

 * @see     Collection

 * @see     Map

 * @see     TreeMap

 * @see     Hashtable

 * @since   1.2

 */

public class HashMap<K,V> extends AbstractMap<K,V>

    implements Map<K,V>, Cloneable, Serializable {

    private static final long serialVersionUID = 362498820763181265L;

    /*

     * Implementation notes.

     *

     * This map usually acts as a binned (bucketed) hash table, but

     * when bins get too large, they are transformed into bins of

     * TreeNodes, each structured similarly to those in

     * java.util.TreeMap. Most methods try to use normal bins, but

     * relay to TreeNode methods when applicable (simply by checking

     * instanceof a node).  Bins of TreeNodes may be traversed and

     * used like any others, but additionally support faster lookup

     * when overpopulated. However, since the vast majority of bins in

     * normal use are not overpopulated, checking for existence of

     * tree bins may be delayed in the course of table methods.

     *

     * Tree bins (i.e., bins whose elements are all TreeNodes) are

     * ordered primarily by hashCode, but in the case of ties, if two

     * elements are of the same "class C implements Comparable<C>",

     * type then their compareTo method is used for ordering. (We

     * conservatively check generic types via reflection to validate

     * this -- see method comparableClassFor).  The added complexity

     * of tree bins is worthwhile in providing worst-case O(log n)

     * operations when keys either have distinct hashes or are

     * orderable, Thus, performance degrades gracefully under

     * accidental or malicious usages in which hashCode() methods

     * return values that are poorly distributed, as well as those in

     * which many keys share a hashCode, so long as they are also

     * Comparable. (If neither of these apply, we may waste about a

     * factor of two in time and space compared to taking no

     * precautions. But the only known cases stem from poor user

     * programming practices that are already so slow that this makes

     * little difference.)

     *

     * Because TreeNodes are about twice the size of regular nodes, we

     * use them only when bins contain enough nodes to warrant use

     * (see TREEIFY_THRESHOLD). And when they become too small (due to

     * removal or resizing) they are converted back to plain bins.  In

     * usages with well-distributed user hashCodes, tree bins are

     * rarely used.  Ideally, under random hashCodes, the frequency of

     * nodes in bins follows a Poisson distribution

     * (http://en.wikipedia.org/wiki/Poisson_distribution) with a

     * parameter of about 0.5 on average for the default resizing

     * threshold of 0.75, although with a large variance because of

     * resizing granularity. Ignoring variance, the expected

     * occurrences of list size k are (exp(-0.5) * pow(0.5, k) /

     * factorial(k)). The first values are:

     *

     * 0:    0.60653066

     * 1:    0.30326533

     * 2:    0.07581633

     * 3:    0.01263606

     * 4:    0.00157952

     * 5:    0.00015795

     * 6:    0.00001316

     * 7:    0.00000094

     * 8:    0.00000006

     * more: less than 1 in ten million

     *

     * The root of a tree bin is normally its first node.  However,

     * sometimes (currently only upon Iterator.remove), the root might

     * be elsewhere, but can be recovered following parent links

     * (method TreeNode.root()).

     *

     * All applicable internal methods accept a hash code as an

     * argument (as normally supplied from a public method), allowing

     * them to call each other without recomputing user hashCodes.

     * Most internal methods also accept a "tab" argument, that is

     * normally the current table, but may be a new or old one when

     * resizing or converting.

     *

     * When bin lists are treeified, split, or untreeified, we keep

     * them in the same relative access/traversal order (i.e., field

     * Node.next) to better preserve locality, and to slightly

     * simplify handling of splits and traversals that invoke

     * iterator.remove. When using comparators on insertion, to keep a

     * total ordering (or as close as is required here) across

     * rebalancings, we compare classes and identityHashCodes as

     * tie-breakers.

     *

     * The use and transitions among plain vs tree modes is

     * complicated by the existence of subclass LinkedHashMap. See

     * below for hook methods defined to be invoked upon insertion,

     * removal and access that allow LinkedHashMap internals to

     * otherwise remain independent of these mechanics. (This also

     * requires that a map instance be passed to some utility methods

     * that may create new nodes.)

     *

     * The concurrent-programming-like SSA-based coding style helps

     * avoid aliasing errors amid all of the twisty pointer operations.

     */

    /**

     * The default initial capacity - MUST be a power of two.

     */

    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**

     * The maximum capacity, used if a higher value is implicitly specified

     * by either of the constructors with arguments.

     * MUST be a power of two <= 1<<30.

     */

    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**

     * The load factor used when none specified in constructor.

     */

    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**

     * The bin count threshold for using a tree rather than list for a

     * bin.  Bins are converted to trees when adding an element to a

     * bin with at least this many nodes. The value must be greater

     * than 2 and should be at least 8 to mesh with assumptions in

     * tree removal about conversion back to plain bins upon

     * shrinkage.

     */

    static final int TREEIFY_THRESHOLD = 8;

    /**

     * The bin count threshold for untreeifying a (split) bin during a

     * resize operation. Should be less than TREEIFY_THRESHOLD, and at

     * most 6 to mesh with shrinkage detection under removal.

     */

    static final int UNTREEIFY_THRESHOLD = 6;

    /**

     * The smallest table capacity for which bins may be treeified.

     * (Otherwise the table is resized if too many nodes in a bin.)

     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts

     * between resizing and treeification thresholds.

     */

    static final int MIN_TREEIFY_CAPACITY = 64;

    /**

     * Basic hash bin node, used for most entries.  (See below for

     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)

     */

    static class Node<K,V> implements Map.Entry<K,V> {

        final int hash;

        final K key;

        V value;

        Node<K,V> next;

        Node(int hash, K key, V value, Node<K,V> next) {

            this.hash = hash;

            this.key = key;

            this.value = value;

            this.next = next;

        }

        public final K getKey()        { return key; }

        public final V getValue()      { return value; }

        public final String toString() { return key + "=" + value; }

        public final int hashCode() {

            return Objects.hashCode(key) ^ Objects.hashCode(value);

        }

        public final V setValue(V newValue) {

            V oldValue = value;

            value = newValue;

            return oldValue;

        }

        public final boolean equals(Object o) {

            if (o == this)

                return true;

            if (o instanceof Map.Entry) {

                Map.Entry<?,?> e = (Map.Entry<?,?>)o;

                if (Objects.equals(key, e.getKey()) &&

                    Objects.equals(value, e.getValue()))

                    return true;

            }

            return false;

        }

    }

    /* ---------------- Static utilities -------------- */

    /**

     * Computes key.hashCode() and spreads (XORs) higher bits of hash

     * to lower.  Because the table uses power-of-two masking, sets of

     * hashes that vary only in bits above the current mask will

     * always collide. (Among known examples are sets of Float keys

     * holding consecutive whole numbers in small tables.)  So we

     * apply a transform that spreads the impact of higher bits

     * downward. There is a tradeoff between speed, utility, and

     * quality of bit-spreading. Because many common sets of hashes

     * are already reasonably distributed (so don't benefit from

     * spreading), and because we use trees to handle large sets of

     * collisions in bins, we just XOR some shifted bits in the

     * cheapest possible way to reduce systematic lossage, as well as

     * to incorporate impact of the highest bits that would otherwise

     * never be used in index calculations because of table bounds.

     */

    static final int hash(Object key) {

        int h;

        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);

    }

    /**

     * Returns x's Class if it is of the form "class C implements

     * Comparable<C>", else null.

     */

    static Class<?> comparableClassFor(Object x) {

        if (x instanceof Comparable) {

            Class<?> c; Type[] ts, as; ParameterizedType p;

            if ((c = x.getClass()) == String.class) // bypass checks

                return c;

            if ((ts = c.getGenericInterfaces()) != null) {

                for (Type t : ts) {

                    if ((t instanceof ParameterizedType) &&

                        ((p = (ParameterizedType) t).getRawType() ==

                         Comparable.class) &&

                        (as = p.getActualTypeArguments()) != null &&

                        as.length == 1 && as[0] == c) // type arg is c

                        return c;

                }

            }

        }

        return null;

    }

    /**

     * Returns k.compareTo(x) if x matches kc (k's screened comparable

     * class), else 0.

     */

    @SuppressWarnings({"rawtypes","unchecked"}) // for cast to Comparable

    static int compareComparables(Class<?> kc, Object k, Object x) {

        return (x == null || x.getClass() != kc ? 0 :

                ((Comparable)k).compareTo(x));

    }

    /**

     * Returns a power of two size for the given target capacity.

     */

    static final int tableSizeFor(int cap) {

        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;

    }

    /* ---------------- Fields -------------- */

    /**

     * The table, initialized on first use, and resized as

     * necessary. When allocated, length is always a power of two.

     * (We also tolerate length zero in some operations to allow

     * bootstrapping mechanics that are currently not needed.)

     */

    transient Node<K,V>[] table;

    /**

     * Holds cached entrySet(). Note that AbstractMap fields are used

     * for keySet() and values().

     */

    transient Set<Map.Entry<K,V>> entrySet;

    /**

     * The number of key-value mappings contained in this map.

     */

    transient int size;

    /**

     * The number of times this HashMap has been structurally modified

     * Structural modifications are those that change the number of mappings in

     * the HashMap or otherwise modify its internal structure (e.g.,

     * rehash).  This field is used to make iterators on Collection-views of

     * the HashMap fail-fast.  (See ConcurrentModificationException).

     */

    transient int modCount;

    /**

     * The next size value at which to resize (capacity * load factor).

     *

     * @serial

     */

    // (The javadoc description is true upon serialization.

    // Additionally, if the table array has not been allocated, this

    // field holds the initial array capacity, or zero signifying

    // DEFAULT_INITIAL_CAPACITY.)

    int threshold;

    /**

     * The load factor for the hash table.

     *

     * @serial

     */

    final float loadFactor;

    /* ---------------- Public operations -------------- */

    /**

     * Constructs an empty {@code HashMap} with the specified initial

     * capacity and load factor.

     *

     * @param  initialCapacity the initial capacity

     * @param  loadFactor      the load factor

     * @throws IllegalArgumentException if the initial capacity is negative

     *         or the load factor is nonpositive

     */

    public HashMap(int initialCapacity, float loadFactor) {

        if (initialCapacity < 0)

            throw new IllegalArgumentException("Illegal initial capacity: " +

                                               initialCapacity);

        if (initialCapacity > MAXIMUM_CAPACITY)

            initialCapacity = MAXIMUM_CAPACITY;

        if (loadFactor <= 0 || Float.isNaN(loadFactor))

            throw new IllegalArgumentException("Illegal load factor: " +

                                               loadFactor);

        this.loadFactor = loadFactor;

        this.threshold = tableSizeFor(initialCapacity);

    }

    /**

     * Constructs an empty {@code HashMap} with the specified initial

     * capacity and the default load factor (0.75).

     *

     * @param  initialCapacity the initial capacity.

     * @throws IllegalArgumentException if the initial capacity is negative.

     */

    public HashMap(int initialCapacity) {

        this(initialCapacity, DEFAULT_LOAD_FACTOR);

    }

    /**

     * Constructs an empty {@code HashMap} with the default initial capacity

     * (16) and the default load factor (0.75).

     */

    public HashMap() {

        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted

    }

    /**

     * Constructs a new {@code HashMap} with the same mappings as the

     * specified {@code Map}.  The {@code HashMap} is created with

     * default load factor (0.75) and an initial capacity sufficient to

     * hold the mappings in the specified {@code Map}.

     *

     * @param   m the map whose mappings are to be placed in this map

     * @throws  NullPointerException if the specified map is null

     */

    public HashMap(Map<? extends K, ? extends V> m) {

        this.loadFactor = DEFAULT_LOAD_FACTOR;

        putMapEntries(m, false);

    }

    /**

     * Implements Map.putAll and Map constructor.

     *

     * @param m the map

     * @param evict false when initially constructing this map, else

     * true (relayed to method afterNodeInsertion).

     */

    final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {

        int s = m.size();

        if (s > 0) {

            if (table == null) { // pre-size

                float ft = ((float)s / loadFactor) + 1.0F;

                int t = ((ft < (float)MAXIMUM_CAPACITY) ?